The ethylene copolymers which are employed in the process of the present invention are those thermoplastic polymeric substances which have been used in the prior art for preparing compositions according to so-called "hot melt" processes. Copolymers of ethylene and vinyl acetate have been prominent in hot melt technology while copolymers of ethylene with acrylate esters have been of lesser importance.
The uses and the compounding of adhesives to be employed in hot melt processes have been reviewed by J. D. Domine and R. H. Schaufelberg in Chapter 30 of the second edition of the "Handbook of Adhesives" edited by Irving Skeist and published by the Van Nostrand Reinhold Company in 1977.
According to the definitions contained therein, a hot melt adhesive is a thermoplastic compound, which is normally solid at room temperature and which becomes sufficiently fluid at elevated temperature to "wet" a substrate to which it is applied. On cooling from the molten condition the compound sets rapidly and forms strong bonds with the substrate. Crosslinking of the polymer is not necessary in order to obtain good physical properties, hence the cycle of melting and solidification can be repeated.
The use of compositions containing ethylene copolymers according to hot melt processes results in superior adhesion to a variety of substrates compared with the results obtained when polyethylene is the principal binder. Although ethylene/vinyl acetate copolymer-containing compositions are more susceptible to cold flow and to solvent attack than those which comprise either polyethylene or a polyamide as the principal binder, nevertheless by reason of their superior adhesion, good mechanical properties such as a high degree of flexibility, and their wide compatibility with a variety of modifier resins and waxes, these copolymers offer a wider scope for the formulation of adhesives and other useful compositions.
Water is absent from compositions which are applied by means of hot melt processes, likewise volatile organic solvents.
When an ethylene copolymer is to be used in a hot melt process, it is generally compounded with suitable modifier resins and/or waxes. Whereas the ethylene copolymer contributes elastomeric properties, which normally influence favourably such physical properties as cohesive strength, flexibility, toughness and adhesion, at both low and elevated temperatures, and contributes to the realization of an acceptable melt viscosity, the modifier resin imparts specific adhesion to the substrate and "wetting" thereof by the composition. Non-polymeric or non-resinous components often added to adhesive compositions applied as hot melts are waxes as mentioned above; liquid plasticizers; inorganic fillers; pigments; antioxidantes etc.
According to the aforesaid chapter in the "Handbook of Adhesives" edited by Irving Skeist, plasticizers or liquid modifiers are used to a limited extent in order to impart such properties as flexibility, specific wetting and viscosity characteristics to the composition. The liquid plasticizers which have been previously proposed for use together with ethylene copolymers in compositions to be applied by hot melt processes belong generally speaking to the class of organic esters, although other liquid substances such as chlorinated polynuclear aromatic compounds have also been proposed. The particular plasticizer and the proportion thereof used in a particular composition depend upon a number of factors, important considerations being the cost and the compatibility of the plasticizer with the other ingredient of the composition, especially with the ethylene copolymer.
In order to indicate the quantitative range of ingredients most often used in the commercial practice of hot melt processes, reference is made to the following table reproduced from Chapter 1 of the "Handbook of Adhesives".
______________________________________ Ingredient Proportion by weight, % ______________________________________ Ethylene copolymer(s) 20-50 Modifier resin(s) 20-50 Wax(es) 0-20 Plasticizer(s) - liquid 0-20 Filler(s) 0-20 Antioxidant(s) 0.1-1 ______________________________________
The same work of reference, on pages 502-504, draws attention to some of the difficulties associated with commercial-scale hot melt processes involving the use of ethylene/vinyl acetate copolymers in the carpet industry, for the following end-uses:
1. double back lamination of tufted carpets, PA1 2. carpet seaming tapes, and PA1 3. peel and stick carpet tapes.
In the case of double back lamination, attention is directed to . . . "the most difficult problem, as yet not satisfactorily solved, (is) applying a high viscosity hot melt to a carpet 15 ft. wide with good uniformity in both machine and transverse directions". After citing and discussing a formulation, the authors comment that: "EVA (ethylene/vinyl acetate) copolymers containing 28 to 33% VA (vinyl acetate) and with a melt index of 5 to 20 have been used for carpet back laminating. These have given the best balance of properties and viscosity. While lower M.I. (melt index) copolymers would have better physical properties, they are much too high in viscosity. Lower VA copolymers are less expensive but have poorer adhesion and flexibility than 28 to 33% VA products" . . . "This application requires a very inexpensive formulation . . . Fillers have to be used sparingly to lower cost because they significantly increase viscosity and reduce flexibility. Since only medium M.I. (5 to 20 dg/min.) copolymers are used, high loadings of filler greatly reduce tensile strength and elongation resulting in a crumbly product".
In addition to the difficulty of applying such large volumes of molten composition to a substrate, there exists also the difficulty of heating and mixing a number of ingredients together in large quantities to form a uniform viscous mass, and of delivering it to the locus of application, whether it be from equipment in an adjacent area of the same plant, or from a different plant altogether.
We have now discovered a process whereby these difficulties can be alleviated or even eliminated, which is of wide applicability for many end-uses for which hot melt processes could be employed according to the prior art, but demonstrating advantages in cases in which large volumes of the composition are being consumed such as in the backing of tufted carpets. This discovery is based upon the discovery that members of a certain class of relatively inexpensive liquid hydrocarbon oils have excellent compatibility with ethylene copolymers after mixtures comprising the latter in powder form together with the hydrocarbon oil have been heated to the melting temperature of the resin.
It is known to be possible to carry out size reduction of certain ethylene copolymers, such as ethylene-vinyl acetate 72/78 copolymers (72% ethylene-28% vinyl acetate) having a low melt index value such as about 5 denoting a relatively high molecular weight compared with copolymers having the same composition and which exhibit a higher melt index. Using equipment and processes known in the prior art it is possible to obtain these copolymers in the form of free-flowing powders. We have now made the valuable and surprising discovery that large proportions of the aforesaid compatible liquid hydrocarbons may be added to an ethylene copolymer in the form of a solid powder utilising simple mixing equipment and without heat.